Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::suboptimal_flops: exponent for bases 2 and e can be computed more accurately

What it meansLooks for floating-point expressions that can be expressed using built-in methods to improve both accuracy and performance. It is in the nursery group and off by default.

What you see

warning: exponent for bases 2 and e can be computed more accurately
 --> main.rs:6:9
  |
6 | let _ = (2f32).powf(a);
  |         ^^^^^^^^^^^^^^ help: consider using: `a.exp2()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#suboptimal_flops
  = note: `-W clippy::suboptimal-flops` implied by `-W clippy::nursery`

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

Negatively impacts accuracy and performance.

New lints still being finished. Off by default and may have false positives.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

use std::f32::consts::E;

let a = 3f32;
let _ = (2f32).powf(a);
let _ = E.powf(a);
let _ = a.powf(1.0 / 2.0);
let _ = a.log(2.0);
let _ = a.log(10.0);
let _ = a.log(E);
let _ = a.powf(2.0);
let _ = a * 2.0 + 4.0;
let _ = if a < 0.0 {
    -a
} else {
    a
};
let _ = if a < 0.0 {
    a
} else {
    -a
};

Write this instead

use std::f32::consts::E;

let a = 3f32;
let _ = a.exp2();
let _ = a.exp();
let _ = a.sqrt();
let _ = a.log2();
let _ = a.log10();
let _ = a.ln();
let _ = a.powi(2);
let _ = a.mul_add(2.0, 4.0);
let _ = a.abs();
let _ = -a.abs();

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
suboptimal_flops = "warn"

In code

#![warn(clippy::suboptimal_flops)]

One run

cargo clippy -- -W clippy::suboptimal-flops

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Nurserygroup

allowdefault level

yesfixed by cargo clippy --fix

1.43.0added in Rust

Clippy's own documentation

What it does

Looks for floating-point expressions that can be expressed using built-in methods to improve both accuracy and performance.

Why is this bad?

Negatively impacts accuracy and performance.

Example

use std::f32::consts::E;

let a = 3f32;
let _ = (2f32).powf(a);
let _ = E.powf(a);
let _ = a.powf(1.0 / 2.0);
let _ = a.log(2.0);
let _ = a.log(10.0);
let _ = a.log(E);
let _ = a.powf(2.0);
let _ = a * 2.0 + 4.0;
let _ = if a < 0.0 {
    -a
} else {
    a
};
let _ = if a < 0.0 {
    a
} else {
    -a
};

is better expressed as

use std::f32::consts::E;

let a = 3f32;
let _ = a.exp2();
let _ = a.exp();
let _ = a.sqrt();
let _ = a.log2();
let _ = a.log10();
let _ = a.ln();
let _ = a.powi(2);
let _ = a.mul_add(2.0, 4.0);
let _ = a.abs();
let _ = -a.abs();

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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